Mechanical power produced by human powered transport is a parameter that reflects both the rider performance and the efficiency of the propulsion system. Monitoring of power output while cycling can help to maintain comfortable and sustainable pedaling over long periods of time. Knowledge of power produced by a rider is very important for people commuting by bike to their work or study and also for those performing fitness exercises. In modern training programmes power measurement is the ultimate tool routinely used by professional coaches to examine performance of elite-level cyclists.
In order to perform power monitoring the vehicle e.g. bicycle, should be equipped with a portable system able to acquire, to calculate and to display data throughout the riding process. After more than three decades of development, a wide range of devices for measuring dynamic parameters in bicycles have been proposed. These devices have been implemented more or less successfully to all moving parts of the bicycle propulsion system, starting from the crank through the chain to the spokes. Operating in various ways, these power meters utilise a similar physical principle: measuring the force applied to one of the moving elements of the power delivery system and the element velocity caused by this force. Force can be measured in power metering systems directly or indirectly—as a momentum produced by this force—a torque. To avoid any inaccuracy in the power measurements an ideal power meter should not be in contact with the propulsion system. In practice, the power converted to the vehicle motion is not equal to the power produced by the human body due to energy loss in transmission elements. In order to measure an actual power the measurement should be performed as close to the power output as possible. For example in bicycles the power output is at the point of contact between the rear wheel tyre and the ground. This property is also important for power meters used on tandems—vehicles driven by more than one cyclist.
Torque measurements are typically performed in the prior art using strain gauges fixed to the pedals, the crank and the chain wheel. International patent application, WO89/000401 to Schoberer (1989) discloses a power meter with deformation elements built in to the chain ring. The deformation is measured by strain gauges and the angular velocity is calculated by counting rotations of the crank. Acquired data is used to calculate the torque created on the crank. However, in order to install this power meter the existing chain ring needs to be removed. As it is produced from precise parts this power meter is expensive and not transferable from bike to bike. Another method to measure power on bicycles is proposed in U.S. Pat. No. 6,356,847 to Gerlitzski (2002). An optical sensor placed on the bottom bracket is used to measure the distance the pedal spindle twists and the angular velocity of the spindle is measured several times per crank revolution. This power meter requires the replacement of the bottom bracket with a specially designed bracket. Due to the specifics of construction the device can be used to measure the power input from the left leg only. A power meter based on the direct measurement of the force exerted in a drive chain is proposed in U.S. Pat. No. 6,356,848 to Cote et al. (2002). The system includes chain vibration and velocity sensors. The frequency of vibration is translated to the chain tension and used with the value of its velocity to calculate the power output. The accuracy of this method is very dependent on correct system installation, which is not straightforward and needs to be performed by skilled personnel. Another power meter based on direct force measurements is disclosed in U.S. patent application 2007/245835 to Hauschildt (2007). The device uses a flexible force sensor inserted in a cycling shoe to record the force applied by the cyclist to the pedal. This data is used in an approximate mathematical model of cycling to calculate an averaging power input from a cyclists' foot. This does not make a measurement of the propulsion system. A power metering system based on the monitoring of the rear wheel twisting is described in U.S. Pat. No. 4,811,612 to Mercat (1989). Angular divergence between the hub and the rim created by torque is detected using a disk and a hoop attached to the hub and the rim respectively. Openings formed in the disk and the hoop pass in front of photosensitive detectors. The signal produced by both detectors is used to compute the torque. The method essentially consists in measuring the angular displacement of a peripheral region of the wheel in relation to its central region. Since on a majority of bicycles this displacement is very small, the signal to noise ratio produced by this device is high. This fact makes the data acquisition process unstable and the method unreliable. To install this power meter to a bicycle requires the mounting of a large and specifically built hoop and disk. The installation must be done by a reasonably skilled mechanic with specific tools for the job. Furthermore, using a bike equipped with the perforated hoop may be hazardous for the rider and for other people around.
It is to the provision of an accurate, transferable, safe, simple and non-expensive device for monitoring torque transmitted by a driving wheel produced by a rider of a human powered vehicle that the present invention is primarily directed. A power meter using the torque measurement is also provided.